Does Light Rail Really Alleviate Highway Congestion?

Transit advocates take for granted that public rail transportation relieves congestion on the roadways, but experts consider the question far from settled. Take two influential studies published in the last few years as an example: One found the latent demand for road space so strong that even expanding public transit can't hope to diminish it, while another concluded that cities with well-established rail systems do indeed have less traffic [PDF] than those that do not. And that's just in the recent past; the debate stretches back much farther, of course.

New research by two geographers at the University of Denver nudges the literature in a hopeful direction. Focusing on light rail in Denver, Sutapa Bhattacharjee and Andrew Goetz examine the question from two angles at once: they perform a temporal analysis that compares highway traffic before and after the system opened, and a spatial analysis that measures whether or not traffic changes have taken root on highways adjacent to the rail corridor. In a paper set for publication in the Journal of Transport Geography, Bhattacharjee and Goetz believe the light rail system, if nothing else, has made the traffic situation better than it would have been:

Overall, the three light rail corridors in operation have succeeded in lowering the rate of increase in the level of traffic on highways within the rail transit influence zone as compared to highways outside the influence zone.

Denver's light rail system consists of three major lines. The Central Corridor, which opened in 1994, serves the downtown area and the city's major sports venues. The Southwest Corridor, which began operation in 2000, serves the Littleton area just south of central Denver. And since 2006 the Southeast Corridor has covered the University of Denver and some major shopping centers nearby.

Bhattacharjee and Goetz first focus on changes in traffic levels, as measured by vehicle-miles traveled, before and after these three lines entered operation. They distinguish between two types of highway traffic: roads inside the rail line's zone of influence, and those outside it. If light rail service really does influence congestion, then traffic on those roads inside the rail corridor should feel the biggest change in traffic.

The changes became most noticeable once the lines had been underway for a year. When the Central Corridor first began, traffic rose 2.6 percent inside the zone and 1.2 percent outside it, but the following year it rose the same amount both inside and outside (7.4 percent). In 1999-2000, when the Southwest entered operation, traffic rose 13 percent inside the rail zone and 3 percent outside it, but the following year it fell more than 4 percent inside the zone while continuing to rise outside. When the Southeast line started, in 2005-2006, highway traffic decreased both inside and outside the rail zone, but while it continued to fall the following year inside the zone, it rose outside it.

Bhattacharjee and Goetz then looked for traffic fluctuations on certain roads adjacent to the various light rail lines. By this metric the impact of light rail was less striking, but still perceptible at times.

While traffic on roads near the Central Corridor increased 2 percent during the rail's third year of service, the bulk of the rise was on a single interstate. Three other adjacent roads experienced a decrease in congestion — one dropping 16 percent. The 13 percent rise on traffic near the Southwest line in 1999-2000 was attributable mostly to an increase on a single road, Santa Fe Drive, which was widened just before the rail began operation. When road traffic near the line fell 4.6 percent the following year, congestion on Santa Fe in particular dropped 9 percent.

Taken altogether, the findings suggest that light rail has had an important impact on traffic in the Denver region. At the very least, traffic has increased less quickly in corridors served by the rail: for instance, during the entire span of the study, from 1992 to 2008, vehicle-miles traveled rose 41 percent outside the light rail zones and only 31 percent inside it. That's particularly impressive considering the vast amount of development along rail corridors during that time, the authors conclude:

The impact of light rail, though not visible on the interstate highways, cannot be considered inconsequential. Light rail kept the rate of increase of traffic lower within the influence zone despite the large amount of residential, office, and commercial developments taking place around the light rail stations. From 1997 to 2010, approximately ‘‘18,000 residential dwelling units, 5.3 million square feet of retail space, 5.4 million square feet of office space, and 6.2 million square feet of medical space were developed within one-half mile of existing and planned transit stations’’ (Ratner and Goetz, 2011). The light rail ridership has increased steadily since the opening of the first corridor in 1994 (Table 7). It increased from 4 million in 1995 to 20 million in 2009, an increase of about 400%. If all the light rail users had driven cars instead, the level of traffic would have been much worse.

About the Author

Eric Jaffe is a senior associate editor at CityLab. He writes about transportation as well as behavior, crime, and history, and has a general interest in the science of city life. He's the author of A Curious Madness (2014) and The King's Best Highway (2010), and lives in New York.